1. Field of the Invention
This invention relates in general to an optical scanning device, a scanning control device for controlling the optical device, a program used by a computer for executing a scanning control, a positioning mechanism for positing the optical scanning device within an image forming device, an optical scanning unit with a plurality of optical scanning devices, and an image forming device or a read device using the optical scanning device. In particular, the invention relates to a vibration mirror used for the optical scanning device and a method for making the vibration mirror.
2. Description of the Related Art
In a conventional optical scanning device, a mirror substrate, which is formed by a thin film that is supported by two rods, is reciprocatively vibrated using the two rods as a torsional rotation axis by an electrostatic attraction between electrodes disposed at a location opposite to the mirror substrate.
In comparison with a conventional optical scanning device with rotation of polygon mirrors using motors, the described optical scanning device, which is formed with a so-called micro-machining technology, can be wholly constructed by a simple semiconductor process, and therefore, the optical scanning device can be easily made very small, the manufacturing cost can be reduced, and an uneven accuracy due to plural faces with a single reverse inclined surface does not exists. Furthermore, an effect capable of corresponding to a high speed for the reciprocation scanning can be expected. Therefore, a device, in which an image device where an optical writing is performed by a electrophotographic process to form an image, or a reading device where an optical scanning is performed to read an image is installed, is proposed and disclosed.
As the torsional vibration type optical scanning device with an electrostatic driving ability, several disclosures are known. For example, in Japanese Patent No. 2924200, the rod is made into a S shape and its rigidity is thus reduced, so that a small driving force and a large vibrational angle are obtained. Also, in the Japanese Laid Open Application No. 7-92409, the thickness of the rod is thinner than a frame substrate. In Japanese Patent No. 3011144, fixed electrodes are arranged at a location not overlapped with the vibrational direction of the mirrors.
Through etching an SOI substrate by dry etching is a widely known method to form a fundamental shape of the vibration mirror that is made by the micro-machining technology, for example, a disclosure taught by Japanese Patent Publication 2001-513223. In this method, an etching mask is formed on a first silicon substrate of the SOI substrate, and then a through etching process for the first silicon substrate is performed to shape the movable mirrors and the torsion rods. Afterwards, a region of a second substrate including the through region is removed by etching, or a region comprising the through region of the second substrate is removed by etching to form the etching mask on the first substrate of the SOI substrate, so that the first substrate is through etched to form the shapes of the movable mirrors and the torsion rods. The dry etching for the first silicon substrate usually uses high density plasma etching with an etching gas of SF6. At this time, an oxide film with a large etching ration to silicon is widely used, and a portion of the oxide film mask formed with a uniform thickness is removed by etching to form a through region. In this method, a fundamental shape can be formed by bringing out the movable mirror as the reflection means holding means and the rod profile as the torsional rotation shaft by penetrating the substrate. However, the thickness of the movable mirror made by the above manner is entirely uniform, and if the thickness of the first silicon substrate of the SOI substrate is directly used as the thickness of the movable mirror, the momentum of inertia of the movable mirror is large and the vibration angle is large, so that a large driving torque is required.
FIG. 54 is a vertical cross-sectional front view showing a conventional vibration mirror 900 and FIG. 55 is a perspective view. The vibration mirror 900 comprises a substrate 905, movable electrodes 904 disposed at the substrate 905, fixed electrodes 906 disposed opposite to the movable electrodes 904. A rotation axis composed of two rods 903a, 903b that are used to support a mirror substrate 902 having a mirror 901 thereon is formed in the substrate 905. Using the rotation axis 903 composed of the two rods 903a, 903b, and the mirror substrate 902 is reciprocatively vibrated by the electrostatic attraction between the movable electrodes 904 and the fixed electrodes 906. In this way, the mirror 901 rotates to change the reflection direction of an incident light to perform the optical scanning.
The magnitude of a vibration angle θ of the mirror substrate 902 is tiny only under the action of the electrostatic driving torque (T1) as shown in FIG. 56. The mirror substrate 902 is driven by a driving pulse that is equivalent to a mechanical resonant frequency of movable parts, i.e., a vibration state. The vibration angle θ of the mirror substrate 902 with the mirror 901 under the resonant state is expressed as θ=(Tq/I)·K, wherein Tq is an electrostatic torque acted onto the mirror substrate 902 with the mirror 901, I is an inertial moment of the mirror substrate 902 with the mirror 901, and K is a constant determined by the rotation axis 903 composed of the two rods 903a, 903b. 
Namely, it is a method that when the vibration angle θ of the mirror 901 on the mirror substrate 902 gets large, the electrostatic torque Tq gets large but the inertial moment I gets small. In particular, when vibration mirror is used in an optical writing device such as a laser printer using a electrophotographic process, it is require to have a mirror on a mirror substrate, wherein the mirror has a reflection plane defined by a beam shape formed on a photosensitive surface. When the mirror face gets large, the mirror substrate also becomes large so that the inertial moment of the mirror cannot be small.
In addition, the resonant frequency f0 and the inertial moment I of the mirror substrate 902 has a relationship of f0 ∝I−1/2. The resonant frequency f0 will deviate due to processing errors of such as the thickness of the mirror substrate, the profile dimension, etc. As described, if the scanning frequency shifts away from the resonant frequency, the vibration angle of performing the optical scanning becomes small. For obtaining a desired vibration angle, the only way is to increase the driving voltage and enlarge the electrostatic torque Tq, so that there are limitations. As a result, in the optical scanning device, the scanning frequency is set according to each resonant frequency.
Accordingly, an error of about several percentages (%) with respect to the design standard occurs, and the main scanning times per unit time, i.e., the scanning frequency is different for each case. As the scanning frequency is different, the size of the formed image in the secondary scanning direction is different. For example, if the resonant frequency is faster than the design standard, the size of the image in the secondary scanning direction becomes small because the pixel pitch in the secondary scanning direction becomes short. If the error is 1%, the size in the sub-scanning direction is shifted by one pixel as being scanned one hundred times in the main scanning direction. In particular, for an image forming device to form one image by overlapping images that are recorded by plural optical scanning devices, image degradation is a severe problem.